The Joke about buying a new bed

The other day I went to the mattress store because I wanted a new bed. I wandered around for a while when a salesman approached me. I told him what was up and he had me follow him into the back room. There we looked at a real nice mattress.
He said to me, "This is the best mattress on the market. Laying on this thing feels like you're just floating in the air."
And that made me think, "How the fuck do astronauts stay awake?"